The present invention relates to equipment and methods for thermal conversion of light hydrocarbons such as natural gas and other reactants to desired end products, particularly diatomic hydrogen and elemental carbon. The end products may be either a gas or ultrafine solid particles. The present invention also relates specifically to methods for effectively producing such end products.
Methane and other light hydrocarbon combustibles are often found in remote areas. Worldwide natural gas reserves have increased on an average of about six percent annually, while natural gas consumption has only increased about three percent annually.
The difference between known and used reserves has increased therefore to about 4.6 quadrillion cubic feet. Known natural gas reserves therefore have an energy equivalent of some 770 billion barrels of oil that is equivalent to about a 29 year worldwide supply of oil for energy purposes. Unfortunately, however, most of the natural gas reserves are located in remote areas. Remote natural gas reserves usually makes the economics of extraction and removal unfeasible.
The Fischer-Tropsch process, developed early in the 20th century in Germany, uses fossil fuels and converts the fossil fuels to liquid synthetic gasoline species. The Fischer-Tropsch synthesis is strongly exothermic and often requires hydrogen in the process. Where a Fischer-Tropsch process is being conducted at remote sites without a proper infrastructure for readily available hydrogen, the cost of production is significantly increased by the need to bring the hydrogen to the remote site.
As environmental concerns increase regarding greenhouse gases that may contribute to global warming, increased interest is directed toward finding clean burning fuels that do not produce carbon dioxide emissions. Hydrogen as a fuel seems to be ideal as it burns to form only water as its combustion product.
A need has long existed for converting available carbonaceous materials to scarce liquid hydrocarbon fuels having preferred performance characteristics in many applications, such as internal combustion engines and other heat engines. Prior art technology teaches converting coal to a liquid hydrocarbon fuel by gasifying the coal to synthesis gas, hydrogenating the resulting synthesis gas, and recovering a liquid hydrocarbon fuel from the hydrogenation product.
A need has long existed for converting available carbonaceous materials to scarce liquid hydrocarbon fuels having preferred performance characteristics in many applications, such as internal combustion engines, jet engines and open-cycle gas turbines. Thus, for example, U.S. Pat. No. 3,986,349 teaches a process for converting solid coal to a liquid hydrocarbon fuel by gasifying the coal to a synthesis gas, hydrogenating the resulting synthesis gas, and recovering a liquid hydrocarbon fuel from the hydrogenation product. The liquid hydrocarbon fuel is used to generate power by relatively clean combustion in an open-cycle gas turbine.
An alternative is to produce natural gas and convert it in the field to a more utilitarian liquid hydrocarbon fuel or liquid chemical product for local usage or for more cost-effective transportation to remote markets. Processes for converting light hydrocarbon gases, such as natural gas, to heavier hydrocarbon liquids are generally known in the prior art. Such processes typically involve the indirect conversion of methane to synthetic paraffinic hydrocarbon compounds, wherein methane is first converted to a synthesis gas containing hydrogen and carbon monoxide followed by conversion of the synthesis gas to synthetic paraffinic hydrocarbon compounds via a Fischer-Tropsch reaction. The unconverted synthesis gas remaining after the Fischer-Tropsch reaction is usually catalytically reconverted to methane via a methanation reaction and recycled to the process inlet to increase the overall conversion efficiency of the process.
Conversion of methane to a synthesis gas is often performed by high-temperature steam reforming, wherein methane and steam are reacted endothermically over a catalyst contained within a plurality of externally-heated tubes mounted in a large fired furnace. Alternatively, methane is converted to a synthesis gas via partial-oxidation, wherein the methane is exothermically reacted with purified oxygen. Partial oxidation using purified oxygen requires an oxygen separation plant having substantial compression capacity and correspondingly having substantial power requirements. Production of the synthesis gas via either of the above-recited methods accounts for a major portion of the total capital cost of a plant for converting methane to paraffinic hydrocarbons.
Autothermal reforming is a lower cost method of converting methane to a synthesis gas. Autothermal reforming employs a combination of partial oxidation and steam reforming. The endothermic heat required for the steam reforming reaction is obtained from the exothermic partial oxidation reaction. Unlike the above-recited partial oxidation reaction, however, air is used as the source of oxygen for the partial oxidation reaction. In addition, the synthesis gas produced by autothermal reforming contains substantial quantities of nitrogen from the inlet air. Consequently, it is not possible to recycle the unconverted components contained in the process tail gas without undesirably accumulating an excess of nitrogen within the process. Production of a nitrogen-diluted synthesis gas via autothermal reforming or partial-oxidation using air followed by conversion of the synthesis gas via a Fischer-Tropsch reaction as disclosed in U.S. Pat. Nos. 2,552,308 and 2,686,195 is, nevertheless, a useful method for obtaining synthetic hydrocarbon liquid products from methane.
U.S. Pat. No. 4,833,170 discloses another example of autothermal reforming, wherein a gaseous light hydrocarbon is reacted with air in the presence of recycled carbon dioxide and steam to produce a synthesis gas. The synthesis gas is reacted in the presence of a hydrocarbon synthesis catalyst containing cobalt to form a residue gas stream and a liquid stream comprising heavier hydrocarbons and water. The heavier hydrocarbons are separated from the water and recovered as product. The residue gas is catalytically combusted with additional air to form carbon dioxide and nitrogen which are separated. At least a portion of the carbon dioxide is recycled to the autothermal reforming step.
Although prior art hydrocarbon gas conversion processes such as disclosed in U.S. Pat. No. 4,833,170 may be relatively effective for converting the light hydrocarbon gases to heavier hydrocarbon liquids, such processes have not been found to be entirely cost effective due to significant capital equipment and energy costs attributable to compression of the inlet air. The power required to compress the inlet air represents the majority of the mechanical power required to operate the process, yet much of this power is essentially lost as unrecovered pressure energy in the residue gas from the process. The inlet air requiring compression contains substantial quantities of nitrogen that remain essentially chemically inert as the nitrogen passes through the process, ultimately exiting the process in the residue gas. Furthermore, although the residue gas has a significant chemical-energy fuel value attributable to the carbon monoxide, hydrogen, methane and heavier hydrocarbon components thereof, the residue gas is very dilute, having a low heating value that renders it very difficult and costly to recover the energy of the fuel value of the residue gas with high efficiency. Thus, it is apparent that a need exists for a more cost effective hydrocarbon gas conversion process.
In the above-mentioned technologies, it is well known that a carbon-containing chemical species will be combusted and the amount of greenhouse gases that are emitted to the atmosphere, namely carbon dioxide, is increased. What is needed in the art is a process for making synthetic fuels from light hydrocarbons that eliminates the emission of greenhouse gases.
Another problem that occurs is the need for cryogenic storage of converted fuels such as hydrogen. Hydrogen storage presents a problem because an on-board system of a hydrogen heat engine for a vehicle has a range of approximately 50 miles with hydrogen stored in pressurized tanks. Typically, a combination of both cryogenic and high pressure hydrogen storage are required in order to contain the hydrogen in a compact enough package to carry as an on-board system.
Several attempts have been made to store hydrogen as a metal hydride in order to lessen the need for both cryogenic and high pressure storage. Metal hydride storage has its own challenges included added weight of the metal and added energy required to separate the hydrogen as the hydride of the metal in order to provide it as the fuel source. What is needed in the art is a process of making synthetic fuels from light hydrocarbons that eliminates the problems of hydrogen storage experienced in the prior art.
Another problem that exists in the prior art is the creation of H2 and soot by use of a plasmatron. Although H2 and soot may be created such as taught by Bromberg et al. in U.S. Pat. No. 5,409,784, non-ultrafine soot particle sizes are irregular, and tend to form agglomerations that are of a size above the 2,000 nm range. Although plasmatrons may produce H2 and soot, the extremely chaotic nature of the production of H2 and soot likely cause the irregular soot particle sizes.
The present invention relates to the formation of diatomic hydrogen and elemental carbon from feed stocks of light hydrocarbons such as natural gas and/or methane.
The inventive process operates by injecting light hydrocarbons such as natural gas and other optional reactants into the inlet end of a reaction chamber and rapidly heating the reactants to produce a hot hydrogen and acetylene product stream which flows toward the outlet end of the reaction chamber. The reaction chamber may have a predetermined length that is sufficient to effect heating of the reactant stream to a selected equilibrium temperature and a preferred equilibrium composition of primarily diatomic hydrogen and unsaturated hydrocarbons such as acetylene.
Upon reaching the selected equilibrium temperature, the desired end product is available to be formed from the product stream as a thermodynamically stable or unstable reaction product at a location adjacent to the outlet end of the reactor chamber. The product stream may be passed through a restrictive convergent-divergent nozzle arranged coaxially within the remaining end of the reactor chamber to rapidly cool the gaseous stream by converting kinetic energy to thermal energy as a result of substantially adiabatic and isentropic expansion as it flows axially through the nozzle to minimize back reactions. A gradual expansion is not sought after as further decomposition of the unsaturated hydrocarbon is desired. The product stream thereby reheats to decompose the acetylene into hydrogen and elemental carbon. Thereby, the desired end product within the flowing gaseous stream is retained. Subsequently, the product stream is cooled and slowed down in velocity. Solid material, namely the elemental carbon, is recovered in any suitable device such as a cyclone.
Preferably the rapid heating step is accomplished by introducing a stream of plasma arc gas to a plasma torch at the inlet end of the reaction chamber to produce a plasma within the reaction chamber which extends toward its outlet end.
An alternate method of this invention uses a virtual convergent-divergent nozzle. This is accomplished by directing one or more streams of particles, droplets, liquid, or gas into the main flow stream of the reactor chamber such that the main reactant flow stream is forced to flow as though a real convergent-divergent nozzle were present. This phenomena occurs because the reduced axial momentum of the directing flow effectively impedes the flow of the main stream, thereby forcing the majority of the main stream to flow around the impeding stream, similar to the flow through the restriction of a conventional converging-diverging nozzle. A similar cooling effect is achieved with the virtual nozzle. Although a rapid expansion is preferred in order to form elemental carbon and diatomic hydrogen by thermal decomposition. The directing or impeding stream(s) can play other roles than merely providing the virtual nozzle effect. In addition to keeping the main flow stream away from the wall, they can interact with the main stream further downstream in various ways to provide, for example, enhanced heat transfer, mixing, chemical reaction, etc. The virtual nozzle effect can also be utilized in combination with a conventional, gradual expansion converging-diverging nozzle to achieve optimal performance of a rapid expansion. To obtain the desired expansion and to cool the desired end products of elemental carbon and diatomic hydrogen, it is preferable to adjust the velocity of the reactants, the quantity of the reactants, the number and position of the supply inlets, and diameter of the reactor chamber.
The present invention converts a predominantly natural gas or other light hydrocarbon stream to diatomic hydrogen and elemental carbon with minor amounts of impurities. With either the inventive convergent-divergent nozzle or the virtual convergent-divergent nozzle, the present invention is particularly well suited for the production of ultrafine solid particles comprising elemental carbon. The rapid conversion of intermediate products to end products allows for the formation of the ultrafine carbon particles. Ultrafine carbon particles formed by the present invention are in a size range from about 10 nm to about 100 nm. Where the ultrafine solid particles are in the preferred size range, the present invention is particularly well suited for the storage of the diatomic hydrogen upon the ultrafine elemental carbon.
Some benefits of the present invention include energy efficiency and economically versatile scalability to a variety of production rates from as low as a few thousand cubic feet per hour or lower, to millions of cubic feet per hour or higher. The present invention also has the benefit of sequestering a significant fraction of carbon from the raw feed stock and diverting it from discharge to the environment as a greenhouse gas. The present invention is also useful for the production of hydrogen and carbon in remote areas that only require a suitable site and an available source of natural gas such as methane. Further any electricity needed for the inventive process may be derived from the natural gas itself.
The present invention also uses hydrogen as a plasma gas in lieu of argon or as a major component compared to argon. Thereby, argon supply, and argon separation from the product gases is not required or significantly reduced in importance.
The present invention also relates to an on-board plasma quench reformer system for hydrocarbon fuel such as a natural gas fuel. In the on-board plasma quench reformer, either liquid natural gas or compressed natural gas is vaporized and converted into hydrogen and a selection of carbon compounds including carbon dioxide, carbon monoxide, and elemental carbon. The hydrogen is then supplied to the internal combustion engine as a reformed fuel""source. The carbon is not combusted, and greenhouse gases are not produced.
It is therefore an object of the present invention to provide a method of converting one or more hydrocarbon reactants in a gaseous stream to an end product that has the form of a gas or ultrafine solid particle by heating the reactant stream in an axially reactor to a preferred dissociation equilibrium of elemental carbon and diatomic hydrogen. It is an object of one embodiment of the present invention to accomplish the formation of diatomic hydrogen elemental carbon by quenching the heated reactant stream and rapidly reheating it in order to decompose any unsaturated hydrocarbons. It is another object of one embodiment of the present invention to accomplish decomposition of unsaturated hydrocarbons through a free expansion or through a rapid expansion of the equilibrium gas.
It is an object of one embodiment of the present invention to provide a system for converting a hydrocarbon stream to diatomic hydrogen and elemental carbon.
It is another object of one embodiment of the present invention to rapidly expand the reactant gases by the injection of other gases into the expansion zone to accomplish a virtual free expansion or a virtual rapid nozzle expansion. It is another object of one embodiment of the present invention to provide a waste heat economizer to preheat the reactant gases for the purpose of increased thermodynamic efficiency. It is another object of one embodiment of the present invention to separate unsaturated hydrocarbons from the end product stream by gas absorption and other separations such acetylene separation from acetone. It is another object of one embodiment of the present invention to provide a hydrogen fuel cell system that receives the diatomic hydrogen that has been converted from the light hydrocarbon feed stream. It is the object of one embodiment of the present invention to provide an on-board system that uses a hydrocarbon fuel such as liquid natural gas or compressed natural gas to convert the light hydrocarbon into elemental carbon and diatomic hydrogen and to supply the diatomic hydrogen to a fuel cell or an internal combustion engine, whereby the elemental carbon is not discharged to the environment as a greenhouse gas.
It is also an object of one embodiment of the present invention to provide a hydrogen storage system that combines the hydrogen and ultrafine carbon solids produced by the inventive method.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.